Nanoelectromechanical systems (NEMS) resonator chemical vapor sensors operate by detecting the sorption of vapor molecules through a shift in resonant frequency. The resonant frequency of NEMS resonators depends both on the mass and stiffness of the device. Upon vapor sorption, both the mass and the stiffness of the device can change. For nanocantilevers, which are one class of NEMS resonators, these factors act in opposite directions with mass loading decreasing the resonance frequency and stiffening increasing the resonance frequency. Further, vapor sorption on the nanocantilever tip is strongly mass loading and minimally stiffening, while vapor sorption at the clamped end is strongly stiffening and minimally mass loading. The resonant behavior of the sensors can be observed using any of static (e.g., displacement) mode or dynamic (e.g., vibratory) mode measurements performed by way of optical frequency sensing, electrical or electronic sensing (for example radio frequency sensing or piezoresistive sensing), or acoustic frequency sensing. It has been shown that NEMS sensors can measure changes in mass at the level of zeptograms (Y. T. Yang, et al., Zeptogram-Scale Nanomechanical Mass Sensing, Nano Lett., Vol. 6, No. 4, 2006).
NEMS chemical vapor sensors are made more sensitive and selective through deposition of polymer films or self assembled monolayers (SAMs). Polymer films are preferred because they can be made with greater thickness and thus the ability to sorb a larger quantity of analyte vapor, which induces a stronger sensor response.
The location and thickness of polymer coatings have been shown to greatly influence sensor response. See, for example, Wright, Y. J. et al., Study of a microcapillary pipette-assisted method to prepare polyethylene glycol-coated microcantilever sensors. Sensors and Actuators B-Chemical 2005, 107 (1), 242-251; Thundat, T. et al., Detection of Mercury-Vapor Using Resonating Microcantilevers. Applied Physics Letters 1995, 66 (13), 1695-1697; and Chen, G. Y. et al., Adsorption-induced surface stress and its effects on resonance frequency of microcantilevers. Journal of Applied Physics 1995, 77 (8), 3618-3622.
Polymer film deposition methods on NEMS sensors heretofore have been limited to the dropcasting or spincoating of dilute polymer solutions that coat the entire substrate with a thin polymer film. The maximum film thickness achievable by these methods is approximately 10 nm, which limits dynamic range in terms of both minimum and maximum detectable levels. Attempts to form thicker films have resulted in NEMS cantilevers that are glued to the underlying substrate and thereby rendered unable to resonate. In addition, with these methods, film homogeneity is difficult or impossible to control, leading to variation in film thickness between adjacent sensors and across the surface of a single sensor. Such variation in thickness decreases the reproducibility of sensor performance. In addition, for applications demanding detection of a target vapor from a very small total sample volume, analyte sorption onto non-sensitive surfaces such as the underlying substrate decreases the number of molecules available for detection via sorption onto NEMS, which limits sensitivity.
Previously, dropcasting or dropcoating dilute polymer solutions was necessary to coat nanocantilevers with thin polymer films. FIG. 1A and FIG. 1B show the limitations on film thickness imposed by these methods.
Inkjet printing, which has been used to deposit polymer films on single microscale cantilevers produces solution droplet on the order of tens of microns. See for example Bietsch, A. et al., Rapid functionalization of cantilever array sensors by inkjet printing. Nanotechnology 2004, 15 (8), 873-880. Microcapillary pipettes have also been used, but suffer from the same droplet size problem. See Wright, 2005. Dip pen lithography is capable of the precision to deposit polymer films on single nanocantilevers, but is a serial process and coating of thousands of sensors would be too time intense to be industrially viable. See for example, Salaita, K. et al., Applications of dip-pen nanolithography. Nat. Nanotechnol. 2007, 2 (3), 145-155.
There is a need for systems and methods to deposit films on nanocantilevers that overcome the above recited limitations.